This invention relates to a numerical control method. More particularly, the invention relates to a numerical control method wherein a machining error which occurs in cutting a corner portion, as well as a radially directed machining error which occurs in cutting an arc, is capable of being held within allowable limits.
A so-called data prereading techniques is used as a method of reading data in a numerical control device. The reasons are as follows. In a method which does not rely upon the prereading technique, a succeeding block of NC data is read each time machining or movement ends. This is followed by a format check, decoding, calculation of an amount of movement (incremental values), and by other preprocessing, after which machining or movement is controlled based upon the succeeding block. With this conventional method, however, machining efficiency declines because movement based on the succeeding data does not start until after the time required for preprocessing. It is for this reason that the above-mentioned data prereading technique has come to be used. With this technique, as illustrated in FIG. 1, when numerically controlled machining based on the current block, e.g., the first block B1, is in progress as indicated at W1, NC data in the succeeding block B2 is preread and preprocessing based on the succeeding block B2 is performed in advance, in parallel with NC machining control W1 based on the current block B1. Then, simultaneous with the completion of NC machining control specified by the current block B1, NC machining control W2 is performed on the basis of the NC data in the succeeding block B2. According to the data prereading method, movement based on the NC data in the succeeding block can therefore be executed immediately without waiting for the completion of preprocessing following movement based on the NC data in the current block. The result is a more efficient machining operation.
When turning at a corner portion or cutting an arc in accordance with numerical control based on the foregoing prereading method, a slight deviation develops between the tool path (the path followed by the center of the tool) and the commanded path, as shown in FIGS. 2 and 3. This is caused by a servo delay, such as an exponential acceleration/deceleration characteristic and the characteristic of the DC servomotor used. In FIGS. 2 and 3, the solid lines indicate the commanded path, and the dashed lines show the tool path. The tool path is dependent upon the following:
(a) feed speeds V1, V2 prior to and following a corner;
(b) corner angle .theta.;
(c) time constant T1 of exponential acceleration/deceleration during cutting; and
(d) time constant T2 of motor used. In other words, the difference between the tool path and commanded path depends upon these parameters. The difference between the tool path and commanded path results in a machining error which is required to be held within allowable limits. To this end, according to the prior art, programming is performed during the creation of an NC tape to set the feed speeds so that the error will fall within the allowable limits. Alternatively, dwell command (G04) is inserted between items of command data corresponding to the blocks on either side of a corner to introduce a dwell period of suitable duration. Consequently, programming becomes extremely complicated and the tape grows to a great length. Also, since machining error generally differs from one controlled machining tool to another, programming must be performed for each machine tool, requiring considerable time and effort for the creation of NC tapes.